[media] v4l2-subdev: add registered_async subdev core operation

V4L2 sub-devices that are registered asynchronously have no way to know
when they have been registration with a V4L2 device but they might need
to take some action after this.

So let's add a callback that can be executed by the V4L2 async core to
allow sub-devices drivers to do any needed initialization that depends
on the registration.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

include/media/v4l2-subdev.h

@@ -179,6 +179,8 @@ struct v4l2_subdev_io_pin_config {
  *		     for it to be warned when the value of a control changes.
  *
  * @unsubscribe_event: remove event subscription from the control framework.
+ *
+ * @registered_async: the subdevice has been registered async.
  */
 struct v4l2_subdev_core_ops {
 	int (*log_status)(struct v4l2_subdev *sd);
@@ -211,6 +213,7 @@ struct v4l2_subdev_core_ops {
 			       struct v4l2_event_subscription *sub);
 	int (*unsubscribe_event)(struct v4l2_subdev *sd, struct v4l2_fh *fh,
 				 struct v4l2_event_subscription *sub);
+	int (*registered_async)(struct v4l2_subdev *sd);
 };
 
 /**